How do I add a Panel to the Rigid Body Panel?
The PHYSICS_PT_rigidbody_panel class is only present in the properties_physics_rigidbody.py script - it is simply a base class to provide common properties to the other panels in that script.

The step you are missing is bpy.utils.register_module(), with the properties you need for the panel you get the following -

import bpy

class PHYSICS_PT_rigid_body_worldmatrix(bpy.types.Panel):
    bl_label = "Rigid Body Coordinates"
    bl_space_type = 'PROPERTIES'
    bl_region_type = 'WINDOW'
    bl_context = "physics"
    bl_default_closed = True

    @classmethod
    def poll(cls, context):
        obj = context.object
        return (obj and obj.rigid_body and
                obj.rigid_body.type == 'ACTIVE' and
                (not context.scene.render.use_game_engine))

    def draw(self, context):
        layout = self.layout

        ob = context.object
        rbo = ob.rigid_body

        row = layout.row()                                                                     
        row.prop(ob, "matrix_world", index=12, text="X")
        row.prop(ob, "matrix_world", index=13, text="Y")
        row.prop(ob, "matrix_world", index=14, text="Z")

if __name__ == "__main__":
    bpy.utils.register_module(__name__)